Spam Filters: How They Work

Statistics show that people send approximately 14.5 billion spam messages every day. This assumes that 45% of email messages that are sent out daily end up as spam. Are you are a business owner? Do you rely on email marketing to reach your prospects and customers? If so, you may find these stats disturbing.

One of the vital goals of every email marketer is to get their emails delivered right in the inboxes of their subscribers. How do you connect with your subscribers and make an impression when your email is in the spam folder? This can only be achieved when spam filters find your emails ‘neat and decent’. To make this happen, you need to first learn how spam filters work. That is exactly what this article will talk about.

Table Of Contents

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What Is A Spam Filter?

How Do Spam Filters Work?

What Do Spam Filters Generally Look For?

Tips To Make Your Email Pass Spam Filters

What Is A Spam Filter?

Internet Service Providers (ISPs) use spam filters to detect unwanted and unsolicited emails. Once they identify a spam message, they block it from reaching the target user’s inbox. 

Spam filters are an industry response to the increasing rates of scams perpetuated by internet fraudsters.

How Do Spam Filters Work?

Spam filters use predefined rules, or algorithms, to go through email messages. They look for emails with features that display the traits of spam-like emails.

The algorithm then calculates the probability of that the message could be spam and assigns each part of the message a value. If the total value exceeds a threshold, it will flag the message as spam.

There are different forms of spam filters, each uses unique criteria to determine whether a message is spam or not.

Some of the available spam filters are:

1. Header filters: Header filters check email headers for suspicious information. This can include forged email address. They also go through sender, recipient, and subject fields too. If the email header has pieces of information that are like spam, it will treat it as such. It checks for misspelt words and other spammy traits in the header.

2. Content filters: They go through the contents for some words that spammers are fond of using. Such words include “100% guarantee,” “act now!” and other words that may make the content look suspicious. How you arrange your content, font size used, and other elements will also be considered.

3. Blacklist filters: Blacklist filters check your domain name or IP address whether it is on a blacklist or not. This is to prevent a blacklisted domain name or IP address from sending messages. Some more complex versions will check the IP address’ reputation.

4. Permission filters: These filters require subscribers to allow the sender to send messages to them. They must have given such permission in advance. So, you can’t send unsolicited messages to subscribers.

5. Rule-based filters: This type of filter uses some criteria defined by the user to detect spam and block it. It goes through the email and looks for phrases or words that can make an email pass as spam. Otherwise known as Heuristic filters, it scans an email and assigns points to all the phrases and words in the content. When it finds suspicious words such as spam trigger words, it assigns them higher points. It gives other terms lower points. When it is done, it sums up the scores. It will consider an email as spam if it scores higher than a specific number and a regular email if it scores below the given number.

6. Challenge-response filters: The challenge-response filters go through the sender’s IP address. They try to find out whether subscribers allow the sender to send messages to them or not.

7. Bayesian Filters: They rank as the most advanced filtering system. It uses mathematical formulas to check email content. It then compares it to the sender’s spam and legitimate emails records. So, it can easily differentiate between spam and valid messages.

These are the common filtering techniques used by spam filters. It is possible to find an ISP using a combination of two or more types of filters in reviewing inbound emails.

Spam filters don’t work the same way. But, they look out for some common attributes of spam in determining whether an email deserves to make it into the inbox or not. 

What Do Spam Filters Generally Look For?

1. IP Address or domain

Spam filters may flag your emails if someone has used your IP address to send spam. Filters go through IP addresses to see what those addresses are used for. When checking the domain name or IP address, they consider several factors. These include sending permanence, sender authentication, and other relevant factors. Sending permanence refers to your IP address’ mailing history. Sending emails to your entire list from a new IP address may have a negative impact on your sending permanence.

So, they treat emails from new domains and IP addresses cautiously. If you change your domains and IP addresses frequently, that could be a red flag and might drop your sender rating.

2. Content

Your email content is another thing a spam filter will check. Some Internet Service Providers (ISPs) flag emails with suspicious or specific images and content. Does your email contain spam trigger words such as money back guarantee, outstanding values, or one-time mailing?  If so, it may be flagged. If your content contains link shorteners, it might also be considered spam.

Other important things spam filters look out for in your content include HTML markup, header, text color, footer, urls, and language. They also consider text-to-image ratio, footers, attachments, and code.

Some filters concentrate on the structure of an email while some will check your IP address for blacklisting. This explains why one viable way to make it into the inbox is creating quality and acceptable email content.

3. Your reputation

Your reputation is another major factor considered when it comes to spam filtering. Heuristics and algorithms are used to calculate sender reputation. They consider scores of parameters and millions of data points to determine your reputation score. This score ranges from 1 to 100. Mailbox providers may use your reputation score to filter emails coming from your domain name or IP address.

4. Subscriber engagement

The advancement in technology has given rise to complex filters that consider how subscribers interact with an IP address to determine whether it is spam or not.

Such filters consider your emails’ open rates. If the open rates are high, the filters consider your campaign to be legitimate.

They also consider subscribers’ response to your emails. If the response is impressive, your reputation will receive a massive boost.

Another important factor is how subscribers react to your emails when they are delivered in the junk folder. If they move the email from the junk folder to their inbox, spam filters take that as a sign that you are not spam.

But, if a large percentage of your subscribers continue to move your emails to the junk folder, your reputation will take a hit. This drives your emails closer to getting flagged as spam.

Tips To Make Your Email Pass Spam Filters

If you are tired of having your emails end up in the spam folder, you need to find a way to get on the good side of spam filters. How can you boost your chances of making it to the inbox and get ISPs to fall in love with your emails? There are a couple of things you can do to stop your emails from getting filtered into the spam folder.

Here are some tips you can put in place when creating your emails. These could make a difference in how ISPs handle your emails:

1. Know your email engagement results

It is important to know almost everything about your email engagement. This requires you to know your Reply rate, Open rate, and other values. This enables you to know your emails’ performance and let you know whether your subscribers are interested in your messages or not.

2. Check your spam score before sending them

Before you hit the “Send” button, check your email and see its spam score. There are several tools you can use for testing. MailMonitor is one the best and most efficient email checker tools right now. You can take away the guesswork and deal with the raw data of what your email looks like and how ISPs will treat it. You can do all this before hitting the send button. You can fix the identified problems, thereby increasing your chances of making it to the inbox.

3. Use fewer links

Spam filters will pay special attention to your email if you insert too many links in the email body. Don’t throw a bunch of irrelevant links in your email content. It is better if you limit the number of links you use. Make sure that hyperlinks and email content match correctly.

4. Relevant and quality content is a must

Keeping your subscribers engaged is another way to increase your open rates. Do you want to have a good relationship with your subscribers? Feed them relevant and top quality content. You need to offer your subscribers real value to keep them happy and eager to hear from you. Without this, they will unsubscribe from your list.

According to a research published by Anthony Helmstetter, irrelevant email is responsible for 67% of unsubscribe cases.

5. Proofread your emails

When sending out emails, you are representing your brand. You need to ensure your message is grammatically correct and error-free. While it is true that typos aren’t unforgivable, spammers intentionally misspell words to trick spam filters. So, spelling mistakes now trigger spam.

6. Use the appropriate font size

The font size you use can also determine whether your email will pass as spam or not. For the best results, use a normal font size. Otherwise, your email may fail spam tests if you use fonts that are too small or too big. Spammers use tiny fonts to hide spam texts in emails. They also use big fonts to make offers. So, these fonts trigger spam filters.

7. Use reasonable image-to-text ratio

While images are good for passing information, don’t use them too much in your email. Spam filters can’t read images. You may use images in your email but ensure you have more than enough text to supplement. Only use images to the context and content.

8. Prune your email list

Inactive subscribers will ruin your reputation. Get rid of them regularly. Some subscribers may be active but suddenly grow cold. If your efforts to revive their interests fail, remove such subscribers and move on. Cold responses from subscribers can trigger spam filters.

9. Ask subscribers to add your address to their whitelist

To ensure that your emails are delivered to their inboxes, ask your subscribers to include your address in their whitelist. Messages from an address in a whitelist will not end up in spam folder. It is also proof that subscribers want to receive your messages. This will increase your reputation before ESPs.

10. Send authenticated emails

Another way to avoid spam filters is to show that your domain and emails are credible before you send them out. Once you authenticate them, spammers can’t use your domain for scamming. ISPs will view your domain and emails as genuine and won’t tag it as spam.

DomainKeys Identified Email (DKIM) and Sender Policy Framework (SPF) are two of the best verification tools you can use for this.

DKIM allows you to select IP addresses that you want to allow to use your domain name to send messages. Since spammers can’t use your IP address, they can’t ruin your reputation. This will also reduce the number of complaints registered against you before ISPs. This will increase your chances of getting your emails right in front of your subscribers.

You can use SPF to send the list of approved senders that can use your domain for sending messages. When a sender wants to use your domain name, the ESP checks the sender against the list of approved senders. If it doesn’t find the user’s address in the record, it blocks them from accessing your domain name.

If you want to boost your delivery and engagement rates, you need to find a way to beat spam filters. The tips shared in this article will help you achieve this. For better results, run your emails through a spam checker tool like Mailmonitor.

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